Oxygen is involved in a wide range of normal metabolic reactions and is essential for the survival of all aerobic organisms, including human beings. Reactive oxygen species (ROS), such as superoxide, are produced in abundance as a byproduct of the incomplete reduction of oxygen that has entered the respiratory chain. Superoxide is the precursor of other damaging oxygen species including hydrogen peroxide, the hypochlorite ion and the hydroxyl radical. Oxidase enzymes in cells such as phagocytes and nitric oxide synthases are other sources of ROS.
While low levels of ROS are present under normal physiological conditions, in excess, ROS can cause oxidative damage to cells and tissues by, for example, oxidizing cellular macromolecules such as nucleic acids, lipids and proteins. Cumulative damage to cells in this manner can result in pathology. Not surprisingly then, oxidative damage has been implicated in a wide variety of diseases and conditions including chronic obstructive lung disorders such as smoker's emphysema, reperfusion damage, neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS), heart attacks, stroke, several autoimmune diseases, and aging.
Regarding the latter, oxidative damage to cellular macromolecules has been postulated to accelerate the aging process and shorten lifespan. For example, the level of oxidized methionine in proteins in an animal has been observed to increase with the age of the animal. Moreover, in Drosophila, greater resistance to ROS via over-expression of superoxide dismutase and catalase has been correlated with longer lifespan, whereas genetic disruption of superoxide dismutase and catalase has been correlated with shorter lifespan.
Although cells have evolved their own enzymatic antioxidant systems (e.g., superoxide dismutase, catalase, and peroxidase) to neutralize ROS, such systems may not function at ideal levels to minimize the rate of aging and the development of disease. Accordingly, there is a clear need for non-naturally occurring compositions and methods that prevent or reduce oxidative damage to cells. One approach to increase the antioxidant activity in cells is to provide cells with compounds that directly scavenge ROS, e.g., vitamins C, E, and A, glutathione, ubiquinone, uric acid, carotenoids, and the like. Such conventional antioxidant compounds, however, lose activity after neutralizing only one or two ROS molecules. They are thus limited by the relatively small quantities of ROS that they can destroy.